The instant invention relates to a process for the production of new, branched polysiloxane-polyoxyalkylene copolymers and to their use as stabilizers in the production of polyurethane plastics.
Polysiloxane-polyoxyalkylene copolymers (hereinafter abbreviated to "PPC") are known and used as stabilizers and cell regulators in the production of foams made by the diisocyanate polyaddition process. Because of their particular chemical structure (i.e., strongly hydrophobic, generally branched polysiloxanes are chemically attached in a variety of different ways to polyoxyalkylene ethers differing from one another in their hydrophilicity), products of this type are able to reduce the surface tension of a foamable mixture to a relatively large extent. As a result, the cells in the process of formation are stabilized during the foaming reaction, so that the blowing gas which is formed during the reaction or which is already present in the mixture develops its full effect and foams having a strictly reproducible density are obtained. In addition, PPC's also improve the homogeneity of the cell structure, and favorably influence the mechanical properties of the foams.
Accordingly, PPC's which are used in both rigid and flexible foams and in both polyester and polyether foams, are important constituents of any foaming formulation. Thus, because of the large number of variants in the foaming process, an equally large number of PPC's are known and used. In spite of this, the generally known compounds to date are not able to solve every problem in the polyurethane foam area. Thus, in the production of foams from polyalkylene glycol ethers and simple polyisocyanates, such as tolylene diisocyanate, the use of PPC is equally as important as, for example, the use of tin catalysts because the foamable mixtures are of a very low viscosity and have to be stabilized if a pore structure is to be obtained. According to more recent discoveries (see, e.g., German Offenlegungsschrift No. 2,129,922), the PPC's currently in use adversely affect the burning properties of the foams, i.e., the foams are inflammable. It can be shown that any reduction in the concentration of PPC is accompanied by an improvement in burning properties and, thus, an optimum in burning properties is obtained when no PPC at all is used. Since as mentioned above, polyalkylene glycol ether foam formulations based on the conventional isocyanates have to be stabilized, it is necessary in the absence of PPC to take specific measures, for example, though the use of modified polyisocyanates of higher functionality and viscosity (see, e.g., German Offenlegungsschrift Nos. 1,929,034 and 2,002,064). However, these particular measures make the foaming process relatively complex and uneconomical. Accordingly, there still exists the problem of finding PPC's which do not adversely affect the burning properties of polyether polyurethane foams, but which nevertheless show the required stabilizing and cell-regulating effect.
Surprisingly, this problem is solved by the polysiloxane-polyoxyalkylene copolymers made available by the present invention.